Method and apparatus for magnetic resonance imaging

ABSTRACT

Magnet assembly for use in medical magnetic resonance imaging includes means for increasing flux generation in the gap region to provide the capability of scanning smaller volume regions of a patient at increased levels of scanning resolution. The means for increasing flux generation is mechanical or electromagnetic, is coupled to each of the polar regions and maintains the gap region sufficiently large and unobstructed to allow for access to the patient by several persons during scanning. Tapered outer walls of the polar region proximate the gap region further enhance accessibility to the patient during scanning.

The present application is a continuation of U.S. application Ser. No.09/794,878, which was filed on Feb. 27, 2001 now U.S. Pat. No. 6,975,117and is a continuation of U.S. application Ser. No. 08/980,079, which wasfiled on Nov. 26, 1997 and issued on Feb. 12, 2002 bearing U.S. Pat. No.6,346,816. Both applications are assigned to the assignee of the presentinvention and are incorporated by reference herein. The presentapplication is also related to U.S. Pat. No. 6,922,055 B1, which issuedon Jul. 26, 2005 and is also assigned to the assignee of the presentinvention.

FIELD OF THE INVENTION

The present invention relates to magnets for medical magnetic resonanceimaging, and more particularly, to such magnets having magnetic zoomcapabilities and an open configuration that allows for use of magneticresonance imaging during surgery.

BACKGROUND OF THE INVENTION

Magnetic resonance imaging techniques are currently used to obtainimages of various portions of an anatomical region of interest. Amagnetic resonance imaging magnet assembly generates magnetic fieldgradients to spatially encode the nuclear magnetic resonance (NMR)signals from an anatomical region which is positioned in the path of thefield gradients. The NMR signals are detected and then processed toobtain images that provide an accurate representation of anatomicalfeatures and soft tissue contrast of the region of interest.

Early magnet assemblies for performing magnetic resonance imaging on apatient required that the patient be positioned in a narrow,substantially enclosed gap region. These magnet assemblies inducedclaustrophobic reactions in the patient and also prevented anotherperson, such as a medical attendant or physician, from having easyaccess to the patient while a region of the patient was scanned toobtain a magnetic resonance image.

Recently, open type magnetic resonance imaging magnet assemblies havebeen developed. These open assemblies have a large gap region forreceiving a patient, are configured to be less confining and also permitgreater access to the patient during scanning. For example, magnetassemblies with open areas on four sides of the patient, such as thosedescribed in U.S. patent application Ser. No. 07/993,072, filed Dec. 18,1992, and U.S. patent application, MRI APPARATUS, Gordon Danby, JohnLinardos, Jevan Damadian and Raymond V. Damadian, filed Nov. 21, 1997,both assigned to the assignee of the present invention and incorporatedby reference herein, have been proposed which provide for imagingvolumes large enough to conduct surgery therein.

Also, magnet assemblies have been configured in the form of a room withonly the polar regions of the magnet visible in the room, such asprojecting from either the horizontal or vertical walls of the room.These magnet assemblies further reduce claustrophobic stress for thepatient and allow others even greater access to the patient duringscanning. See Ser. No. 07/993,072. In particular, these magnetassemblies provide that one or more persons can have access to thepatient while the patient is positioned between the poles of the magnetassembly during scanning. This accessibility enables a physician toperform surgical procedures on the patient that are guided by the imagesobtained from scanning of a desired anatomical region of the patient.The images obtained using open magnet assemblies, however, are notnecessarily of a desired resolution to be useful for guiding surgery inan anatomical region, which generally is smaller than the anatomicalregion that the magnet assembly is scanning.

Therefore, there exists a need for an open magnet assembly for magneticresonance imaging which allows several persons to have access to apatient while the patient is undergoing scanning and furthermoreprovides a capability of increasing the resolution of scanning over amore limited region of interest of the patient, as desired, simply andconveniently while maintaining access to the patient substantiallyunimpeded and without requiring that the patient be moved.

SUMMARY OF THE INVENTION

In accordance with the present invention, a magnet assembly for use inmedical magnetic resonance imaging provides a sizable gap region inwhich a patient can be received and allows for substantially unimpededaccess to the patient while the patient is undergoing scanning of anyregion of interest. The magnet assembly has a capability to scan a firstrelatively large volume region of the patient at a first scanningresolution and to scan a second, smaller volume region of the patient athigher scanning resolutions than the first scanning resolution.

In a preferred embodiment, the magnet assembly comprises a ferromagneticyoke configured as a frame and conformed to the structure of an ordinaryroom. The frame includes a pair of opposing vertical ferromagneticelements and a pair of opposing pole supports, each of which forms oneside of the frame, which is the flux return path. The pole supportssupport respective ferromagnetic poles which face each other and areaxially aligned with each other. Each of the poles includes a first bodyportion which is adjacent to the pole support and has a rectangular boxstructure. Each of the poles further includes a second body portionwhich extends away from the first body portion and terminates at a gapfacing surface. The second body portion is a trapezoidal box structurewhich includes opposing walls which extend from and are in the sameplane as the longer sides of the rectangular first body portion andtapered walls which extend towards the center of the pole at the sameangle with respect to the shorter walls of the first body portion. Thefacing surfaces of the respective poles are spaced apart to define a gapregion therebetween for receiving a portion of a patient and each have amagnet field gradient coil support mounted thereto. The gap region andthe tapered walls of the poles which are in proximity to the gap regionprovide for open access to the patient during scanning.

In one aspect of the invention, means for increasing magnetic fluxgeneration in the gap region is coupled to each of the poles. Suchincreasing magnetic flux generation means, or magnetic zoom means,allows for higher resolution scanning of a smaller volume region of apatient in comparison to the scanning resolution and the volume regionof the patient which would be scanned, respectively, when the magneticzoom means is not utilized. Although the magnetic zoom means in themagnet assembly decreases the distance between the facing surfaces ofthe structures of the magnet assembly which extend furthest from therespective poles into the gap region, or the gap distance of the magnetassembly, during higher resolution scanning and, alternatively, alsoduring scanning without magnetic zoom, access to the patient is notsubstantially impeded.

The magnetic zoom means comprises a mechanical magnetic zoom means or anelectromagnetic magnetic zoom means, or both, and either of thesemagnetic zoom magnetic zoom means can be provided in the magnet assemblyaxially or non-axially axially symmetrical about the center of thepoles. The mechanical magnetic zoom means is a ferromagnetic structurewhich extends or is extendible from the facing surface of each pole intothe gap region. The electromagnetic magnetic zoom means comprises asupport containing a distribution of conducting coils which is coupledto the facing surface of each pole and extends or is extendible into thegap region.

In a preferred embodiment of either magnet assembly, each pole includesa hollowed cylindrical region in which a piston formed fromferromagnetic material is received in tight fitting relation to thesurface of the pole which defines the hollowed region. The piston iscoupled to a magnetic zoom operating assembly which is coupled to theadjoining pole support. The operating assembly can position each of thepistons simultaneously and identically at a plurality of positionsextending into the gap region to provide for higher resolution scanningof a more limited volume region of the patient in comparison to theregion which the facing surfaces of the poles define. The facing endsurfaces of the pistons define the more limited volume region. Thesurfaces of the pole and the piston which face each other remain insubstantial contact with each other at all times to provide asufficiently large flux contact area.

In a further preferred embodiment, the hollowed cylindrical region ofeach of the poles receives a first ferromagnetic piston having ahollowed cylindrical region and a second ferromagnetic piston which isdisposed in the hollowed region of the first piston. The first piston isin tight fitting relation to the surface of the pole defining thehollowed region and to the outer surface of the second piston facing thefirst piston. The first and second pistons are each coupled to themagnetic zoom operating assembly. The operating assembly canindependently position each of the first and second pistonssimultaneously and identically, respectively, at various distancesextending into the gap region to provide for higher resolution scanningof a more limited region of a patient and adjustability of the magnetfields within the gap region when the higher resolution scanning isperformed. The surfaces of the pole and the first piston which face eachother, and the surface of the first piston and the second piston whichface each other, remain in substantial contact with each other at alltimes to provide a sufficiently large flux contact area.

In a further embodiment, a multiple axis patient bed is located in thegap region to provide that the patient can be positioned at almost anydesired angle in relation to the facing surfaces of the poles.

In another aspect of the invention, independent electromagnetic zoommeans are positioned within the gap region by a mechanical support meansand are separate and independent from the poles of a magnet assembly.The independent electromagnetic zoom means are arranged in the gapregion to define a volume region of the patient through which anincreased magnetic flux density is directed to provide for higherresolution scanning in that region.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the present invention will be apparentfrom the following detailed description of the presently preferredembodiments, which description should be considered in conjunction withthe accompanying drawings in which:

FIG. 1 is a plan view of an exemplary open entry magnetic resonanceimaging magnet assembly configured in the form of a room according tothe present invention;

FIG. 2A is a vertical section of the magnet assembly in the room takenalong the section line 2A-2A in FIG. 1;

FIG. 2B is a vertical section of the magnet assembly in the room takenalong the section line 2B-2B in FIG. 1;

FIG. 3 is a view of the vertical section of the top polar region of themagnet assembly as shown in FIG. 2B including a mechanical magnetic zoommeans in the form of a ferromagnetic piston;

FIG. 4A is a view of the vertical section of an alternative embodimentof the top polar region of the magnet assembly as shown in FIG. 2Bincluding a mechanical magnetic zoom means in the form of two concentricferromagnetic pistons;

FIG. 4B is a view similar to that of FIG. 4A with the centers of the twopistons shifted away from the symmetrical axis of the polar region;

FIG. 5A is a view of the vertical section of the top polar region of themagnet assembly as shown in FIG. 2B including an electromagneticmagnetic zoom means;

FIG. 5B is a view similar to that of FIG. 5A with the electromagneticmagnetic zoom means positioned further into the gap region;

FIG. 6 is a view of the magnetic field gradient coil support of FIG. 5Ataken along the section line 6-6;

FIG. 7 is a view similar to the that of FIG. 3 having an electromagneticmagnetic zoom means coupled to the piston; and

FIG. 8 is a view of the vertical section of the top and bottom polarregions of the magnet assembly as shown in FIG. 2B including independentelectromagnet magnet zoom means positioned in the gap region and notmechanically connected to the polar regions.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A magnet assembly in accordance with the present invention is configuredto define a sufficiently large gap region which allows several medicalpersonnel to have easy access to a patient positioned in the gap regionwhile an anatomical region of the patient is scanned to obtain magneticresonance images. The scanning can include scanning of a first volumeregion of the patient at a first scanning resolution and utilizing amagnetic zoom capability of the magnet assembly, which increasesmagnetic field strength in the gap region, to scan a volume region ofthe patient, Which is smaller than the first volume region, at a higherscanning resolution than the first scanning resolution.

FIG. 1 illustrates a perspective view of an exemplary magnetic resonancemagnet assembly in a surgery room 10, which provides the capability ofscanning various anatomical regions of interest of a patient in the room10 at a plurality of scanning resolutions for generating magneticresonance images in accordance with the present invention. The magnetassembly, preferably, is a room size magnet and the room 10 is amagnetic resonance imaging operating room in which surgical procedureson a patient can be guided by magnetic resonance images. It is to beunderstood that a magnetic assembly having the magnetic zoom capabilityof the present invention can have other suitable configurations which donot conform to the shape of a room.

Referring to FIG. 1, the magnet assembly includes a ferromagnetic upperpole support 12 and a ferromagnetic lower pole support 14. Ferromagneticelements 16 and 18 are disposed between and at the ends of the polesupports 12 and 14. The ferromagnetic elements 16 and 18 support theupper pole support 12 above the lower pole support 14. The pole supports12 and 14 and the ferromagnetic elements 16 and 18, thus, form foursides of a rectangular ferromagnetic yoke or frame, which is the fluxreturn path.

Preferably, each of the ferromagnetic elements 16 and 18 is a steel slabcomprised of multiple sections about nine feet tall, about ten feet wideand about one foot thick, and each of the pole supports 12 and 14 is asteel slab comprised of multiple sections about sixteen feet long, aboutten feet wide and about one foot thick. Consequently, the upper polesupport 12 lies approximately nine feet above the lower pole support 14and the inwardly facing surfaces of the ferromagnetic elements 16 and 18are spaced apart from one another by a distance of approximatelyfourteen feet.

Ferromagnetic gusset plates 20 are provided at the corners of the frameformed by the ferromagnetic elements 16 and 18 and the pole supports 12and 14. The gusset plates 20 reinforce the frame.

Referring to FIG. 1 and also to FIGS. 2A and 2B, which show verticalcross-sections of FIG. 1 at the lines 2A-2A and 2B-2B, respectively, themagnet assembly further comprises an upper ferromagnetic pole 22 whichprojects downwardly from the upper pole support 12 and a lowerferromagnetic pole 24 which projects upwardly from the lower polesupport 14. Both of the poles 22 and 24 and the pole supports 12 and 14are aligned axially and also symmetrical about an imaginary medial planeS which extends between the ferromagnetic elements 16 and 18. The poles22 and 24 further define a polar axis 26 which extends between the polesupports 12 and 14 and about which the poles 22 and 24 and the polesupports 12 and 14 are symmetrical.

The poles 22 and 24 as illustrated in FIGS. 1, 2A and 2B are coveredwith a shroud (not shown) which substantially conforms to thegeometrical structure of the outer facing surfaces of the poles 22 and24. A more detailed description of the elements contained within orwhich can be associated with the top and bottom polar regions of themagnet assembly, in accordance with present invention, is provided belowin connection with the description of FIGS. 3, 4A, 4B, 5A, 5B, 7 and 8.For clarity, the poles 22 and 24 are described at this point only interms of their outer surfaces, which would be visible to a person in theroom and to which the shroud would substantially conform when the magnetassembly is not utilized to obtain images using its magnetic zoomcapability.

Referring again to FIGS. 1, 2A and 2B, the pole 22 includes arectangular box shaped body portion 28 which is adjacent to the polesupport 12. The body portion 28 includes shorter outer side walls 31which are parallel to the ferromagnetic elements 16 and 18 and longerouter side walls 32 which are orthogonal to the ferromagnetic elements16 and 18. The pole 22 further includes a trapezoidal box shaped bodyportion 30 which is integral with and extends downwards towards theopposing pole 24 from the body portion 28. The body portion 30 includesopposing longer walls 33 which are in the same plane as and extend fromthe side walls 32 of the body portion 28. The body portion 30 furtherincludes opposing tapered outer side walls 34, each of which extendstowards the polar axis 26 at the same angle in relation to the walls 31of the body portion 28. The taper of the walls 34 accordingly decreasesthe lengthwise dimension of the outer walls 33 as the outer walls 33extend away from the body portion 28. The outer walls 33 and 34 of thebody portion 30 terminate in the same plane, which is parallel to theplane S, to form a rectangular pole tip facing surface 38. The facingsurface 38, for example, can have a length about equal to 72 inches anda width about equal to 48 inches.

It is to be understood that the pole 22 can be constructed so that thefacing surface 38 has another shape, such as circular or elliptical, andthat the body portions 28 and 30 would be constructed accordingly toobtain such shape and also to maintain a pair of opposing walls whichface the ferromagnetic elements 16 and 18 and each taper towards thepolar axis 26. The tapered walls 34 of the pole 22 are suitably angledin relation to the polar axis 26 to maximize access to a patient 56which is received in the gap region 42 between the poles 22 and 24 ofthe magnet assembly.

The pole 24 is identical in construction to the pole 22, with likestructures having like reference numerals, and is not described indetail below. For clarity of reference, the facing surface on the pole24 is designated below with the reference 40. The facing surfaces 38 and40 of the poles 22 and 24, respectively, define a magnet gap region 42therebetween which is sufficiently large to receive the body of apatient. For ease of reference, a gap distance is referred to below asthe distance between the surfaces of the polar regions of the magnetassembly which extend furthest into the gap region 42 towards the planeS. Also, a pole separation distance is referred to below as the distancebetween facing surfaces of ferromagnetic structures, such as thesurfaces 38 and 40, of the respective polar regions.

Apertures 44 and 46 are defined in the pole supports 12 and 14,respectively. A magnetic zoom operating assembly 48 is coupled to thesurfaces of the pole supports 12 and 14 which oppose the gap region 42.The assembly 48 covers the apertures 44 and 46. The structure andoperation of the operating assembly 48 in relation to the apertures 44and 46 and a mechanical flux generation increasing means, or so-calledmechanical magnetic zoom means, which can be coupled to each pole of themagnet assembly to provide magnetic zoom capability, is discussed ingreater detail below in connection with FIGS. 3, 4A and 4B.

An upper electromagnetic coil 50 encircles the pole 22 at the junctureof the body portion 28 with the upper pole support 12. A correspondinglower electromagnetic coil 52 encircles the pole 24 at the juncture ofits body portion 28 with the lower pole support 14. The coils 50 and 52,alternatively, can be resistive or superconductive.

The gap region 42 further includes a patient support or bed 54 which atleast a portion of is positioned between the surfaces 38 and 40 and onwhich the patient 56 is positioned lying down. One or more radiofrequency (RF) transmitting and receiving antennae 59 are also includedin the gap region 42, preferably in proximity to a region of interest ofthe patient 56 which will be scanned for obtaining magnetic resonanceimages.

The poles 22 and 24, the coils 50 and 52, the antennae 59, the operatingassembly 48 and electronic components which are coupled to the poles 22and 24, such as magnetic field gradient coils, are linked to aconventional magnetic resonance imaging system 58. The system 58includes elements such as a DC power supply for energizing the coils 50and 52, a gradient coil power supply for energizing the magnetic fieldgradient coils and RF transmitters and receivers which are linked to theantennae 59. The system 58 further includes magnetic resonance imaginghardware and software, such as a microprocessor linked to a memory, thattransforms the magnetic resonance signals detected from a region ofinterest which is scanned into magnetic resonance images. Further, animage display or image data download device, such as a video monitor 60,is connected to the microcontroller in the system 58 and suitablymounted inside the interior of the room 10 so that a physician 62 oranother attendant (not shown) who may be involved in performing medicalprocedures, such as surgery, on the patient 56 and is standing at leastpartially in the gap region 42 near the patient 56, can observe themagnetic resonance images of the patient 56 in real time, whileperforming the medical procedures.

Control apparatus 64, such as a keyboard, joystick, mouse or speechrecognition control module, is also linked to the system 58, such as byhardwire or infrared radiation link, and disposed as near to the patient56 as suitable. The control apparatus 64 allows the physician 62, fromwithin the room 10, to control the type of scanning performed on thepatient 56 and, in particular, to utilize the magnetic zoom capabilityof the magnet assembly to obtain higher resolution scanning of asmaller, more defined volume region of the patient 56 than the regionscanned when the magnetic zoom capability is not utilized. In addition,the monitor 60 can include touch-sensitive elements that similarly allowone to control the type of scanning that the magnet assembly performs.Such computer control elements are well known in the magnetic resonanceimaging art and are not described further herein.

The room 10 further includes a raised floor 66 which is supported abovethe lower pole support 14 by a set of braces 68. The floor 66 extendsover the top of the coils 52 and around the body portion 28 of the pole24. Ceiling suspension support members 72 suspend a ceiling 70 beneaththe upper pole support 12. Wall coverings 74 cover the inwardly facingsurfaces of the ferromagnetic elements 16 and 18 and other walls 76which define the room 10. The floor 66, the ceiling 70 and the wallcoverings 74 preferably are formed from non-magnetic materials such aspolymeric materials, wood fibers, paper and cementitious materials suchas concrete, plaster, plasterboard and the like. The exposed, inwardlyfacing surfaces of the floor 66, the walls 74 and the ceiling 70desirably are formed from standard architectural materials and have theappearance of ordinary room walls. The floor 66 may be continuous with afloor 78 of a building in which the room 10 is located. The wallcoverings 74 may be continuous with the walls 76 of the building.Likewise, the ceiling 70 may be continuous with a ceiling (not shown)which is part of the building.

Thus, the space within the magnet assembly and enclosed by the floor 66,the ceiling 70 and the wall coverings 74 constitutes part of an ordinaryroom, i.e., the room 10. The frame of the magnet assembly, which isdefined by the pole supports 12 and 14 and the ferromagnetic elements 16and 18, is disposed outside of the room 10. Also, the coils 50 and 52are disposed outside of the room 10. The patient 56 or another personinside of the room 10 sees the poles 22 and 24 protruding into the room10 from the ceiling 70 and the floor 66, but otherwise considers theroom 10 to be an ordinary room. The shrouds which cover and conceal thepoles 22 and 24 desirably are formed from non-magnetic materials, suchas polymeric materials. Thus, a patient perceives the magnetic resonanceimaging magnet assembly as entirely open and non-claustrophobic.

Each of the ferromagnetic elements 16 and 18 is disposed about sevenfeet from the polar axis 26 as measured from the polar axis 26 to anyferromagnetic element in a direction perpendicular to the polar axis 26.The disposition of the ferromagnetic elements 16 and 18 at a substantialdistance from the polar axis 26 allows an adult human patient to bepositioned on the support 54, such as a five-axis bed, in a generallyhorizontal position with her body extending along the medial plane S.The bed 54, preferably, can be translated, as seen from the perspectiveof FIG. 1, in any direction in a plane orthogonal to the flux elements16 and 18 and also orthogonal to the facing surfaces 38 and 40. The bed54 also can be rotated up to 360° in either direction in a planeparallel to the plane S and clockwise or counterclockwise about an axisof rotation defined by a line extending between and orthogonal to theferromagnetic elements 16 and 18. Thus, a patient can be disposed in anyradial direction with any part of her body in relation to the surfaces38 and 40, and essentially any part of a normal human patient can beimaged.

Moreover, the space around the poles 22 and 24, as enabled by thetapering of the walls 34, provides an unobstructed working spacesufficient to accommodate the physician 62 or one or more persons, suchas other physicians, nurses or attendants. This space is unobstructed byany portion of the frame of the magnet assembly and extends entirelyaround the poles 22 and 24 and the polar axis 26. Thus, apart from anyobstructions that the patient support 54 or the patient 56 herself cancreate, the attendants can have access to the patient 56 from alldirections. This working space extends to the region of the magnetassembly between the coils 50 and 52, which includes the portion of theworking space disposed above the lower coil 52 and below the upper coil50. The tapered walls 34 of each of the poles 22 and 24 alsoadvantageously provide additional working space in the vicinity of thepatient 56. As such, the magnet assembly affords a degree of access tothe patient 56 that is essentially the same as the degree of accessprovided in an ordinary operating room, with only a slight obstructioncaused by the poles 22 and 24 themselves.

The room 10 also, preferably, is surrounded with a continuous orsubstantially continuous electrically conductive shield, commonlyreferred to as a Faraday shield, which shields the working space and thegap region 42 from radio frequency interference to prevent interferencewith the magnet resonance imaging procedure. The pole supports 12 and 14and the ferromagnetic elements 16 and 18 of the magnet frame areelectrically conductive and thus, individually, form portions of theFaraday shield. The floor 66, the walls 76 and the ceiling 70 of theroom 10 are provided with conductive elements, such as conductive mesh80, as shown in FIG. 1. The conductive mesh 80 may be electricallyconnected to the frame of the magnet assembly by a wire or bonding strap(not shown), which connects the mesh 80 to the frame.

A door 82 and a window 84 of the room 10, each of which penetrates oneof the walls 76, are also provided with conductive coverings, such as amesh in the door 82 and a conductive film on the window 84. Theseconductive coverings desirably are also connected to the remainder ofthe Faraday shield.

The equipment disposed inside of the room 10, and hence inside of theFaraday shield, are suitably designed for low radio frequency (RF)emission. For example, the video monitor 60 may be provided with anenclosure having a conductive shield which is grounded to the frame.Also, fixtures such as overhead lights (not shown) that are secured tothe ceiling 70 may be provided with similar shielding. Equipment forperforming medical procedures on a patient or any other type ofconventional medical equipment also may be disposed inside the room,within the interior of the magnet frame.

In ordinary or normal mode operation of the magnet assembly, in otherwords, when the magnetic zoom capability of the magnet assembly is notutilized in accordance with the present invention, the pole supports 12and 14, the ferromagnetic elements 16 and 18 and the poles 22 and 24 arearranged to provide a path of low magnetic reluctance for the flux thatthe coils 50 and 52 generate. The flux is relatively concentrated in thepoles 22 and 24 and in regions of the upper and lower pole supports 12and 14 adjacent to the polar axis 26. Thus, the magnetic fieldachievable in the gap region 42 at a volume region of the patient 56defined by the area of the surfaces 38 and 40 facing the plane S, in thenormal mode of the magnet assembly, is limited by magnetic saturation ofthe ferromagnetic material in the magnet assembly and the poleseparation distance. In the normal mode, the pole separation distance isthe distance between the surfaces 38 and 40 and is, preferably, equal toabout 36 inches.

In accordance with present invention, means for increasing fluxgeneration in the gap region 42 is coupled to each of the poles 22 and24 to provide a high resolution scanning mode of operation of the magnetassembly, or a so-called magnetic zoom mode, that allows for higherresolution scanning of a smaller region of the patient, in comparison tothe region scanned and the scanning resolution attainable under thenormal mode of operation of the magnet assembly.

In one aspect of the invention, a mechanical means for increasing fluxgeneration in the gap region 42 is coupled to each of the poles 22 and24 of the magnet assembly. FIG. 3 illustrates an exemplary embodiment ofthe magnet assembly including a mechanical magnetic zoom meanscomprising a ferromagnetic piston 88, which can be extended into the gapregion 42 from the poles 22 and 24 in the magnetic zoom mode. FIG. 3shows the piston 88 in the magnet assembly from the perspective of avertical cross-section through the top polar region of the magnetassembly, which includes the top plate support 12, the top pole 22 andthe electromagnetic coil 50, as shown in FIG. 2B. It is noted that thebottom polar region, which includes the plate support 14, the bottompole 24 and the electromagnetic coil 52, would have a structure that isidentical to the top polar region and that the top and bottom polarregions are symmetrically aligned about the polar axis 26 andsymmetrical about the plane S. Therefore, for conciseness, only the toppolar region is described in detail below.

Referring to FIG. 3, the body portions 28 and 30 of the pole 22 arehollowed axially symmetrically about the polar axis 26 to a constantdiameter W to define a hollow cylindrical volume region 86 within thepole 22. The hollowed region 86 extends lengthwise through the entirepole 22, from the surface 38 to the surface of the pole support 12adjacent to the body portion 28, the distance between the former andlatter being equal to t. The aperture 44 in the pole support 12 also hasbeen hollowed about the polar axis 26 to the same constant diameter W todefine a hollow cylindrical volume region extending through the entirethickness of the support 12. The inner surfaces of the pole support 12which define the aperture 44 and the inner surfaces of the pole 22 whichdefine the region 86 are, therefore, aligned with each other.

The hollowed region 86 contains the piston 88. The piston 88 is in theshape of a cylinder bounded lengthwise by an end surface 90 which facesthe assembly 48 and an end surface 92 which faces the gap region 42. Theouter surface of the piston 88 has a constant diameter equal orsubstantially equal to W and the distance between the end surfaces 90and 92 is equal to L. Thus, the outer surface of the piston 88 has aconstant diameter which is substantially equal to the diameter W of theregion 86 and the aperture 44.

An annular ferromagnetic structure called a shim bar 94 is disposed onthe surface 38. The shim bar 94 is mounted at the outer perimeter of thesurface 38 and has a beveled inner surface which faces the pole center.The shim bar 94 is a conventional component which is around theperiphery of the pole 22 and compensates for normal magnetic field falloff at the periphery, thereby increasing the volume of uniform andhomogenous magnetic field in the gap region 42.

An insulative support 96 is mounted on the portion of the surface 38which the shim bar 94 circumscribes. The support 96 is of the samethickness as the shim bar 94 and contains magnetic field gradient coils98 which can conduct electrical current and develop magnetic fieldgradients to spatially encode the region of interest being scannedaccording to well known techniques that are not a part of thisinvention.

An insulative support 100 is mounted on the surface 92 of the piston 88.The support 100 contains magnetic field gradient coils 102 which canconduct electrical current and develop magnetic field gradients. Thesupport 100 with the coils 102 has the same thickness as the support 96,and operates in the same manner as the support 96 with the coils 98. Thesupport 100 and 96 are each electrically coupled (not shown) to thesystem 58 and are independently controllable by the system 58.

Ends 106 of two connecting rods 104 are each rigidly secured to the endsurface 90 of the piston 88. The connecting rods 104 extend from the endsurface 90, through the aperture 44 and are connected at opposite ends108 to a means for piston positioning 110 which is contained in themagnetic zoom operating assembly 48.

Encircling the rods 104 adjacent to the ends 108 are stop means orcylinders 109 which are rigidly connected to the rods 104. The stopcylinders 109 have a diameter which is wider than the apertures in thepiston positioning means 110 through which the rods 104 pass. Reinforcedsupports 119 rigidly mount the piston positioning means 110 to thesurface of the assembly 48 which opposes the plane S.

The piston positioning means 110 is compartmentalized into two chambersby piston head 117. The assembly 48 further includes a controllablepiston actuating means or pump 112 which is coupled to the two chambersof the piston positioning means 110 via the lines 114 and 115,respectively. The piston head 117 and all penetrations of the pistonpositioning means 110 and the pump 112, such as the lines 114 and 115,have air tight seals.

In a preferred embodiment, the combination of the piston positioningmeans 110, the pump 112 and the lines 114 and 115 constitutes aconventional hydraulic positioning device that is controllable bycontrol signals that a microcontroller, such as a microcontroller in thesystem 58, transmits to the pump 112. The pump 112 can control fluidflow over the lines 114 and 115 to maintain the rods 104 at, or to movethe rods 104 to, a predetermined position in relation to the plane S.The positioning means 110 is a conventional hydraulic support which canmaintain the rods 104 stationary or move them towards or away from tothe plane S, based on the fluid that the pump 112 supplies to orreceives from either of the chambers of the positioning means 110.

Based on the control signals transmitted to the pump 112, the pump 112can operate to receive a predetermined amount of fluid from thepositioning means 110 over the line 114 and supply a predeterminedamount of fluid to the positioning means 110 over the line 115 so as toretract the connecting rods 104 into the positioning means 110 apredetermined length, thereby causing the piston 88 to be moved thepredetermined length away from the plane S. On the other hand, theactuating means 112 can operate to supply a predetermined amount offluid under pressure to the positioning means 110 over the line 114 andreceive a predetermined amount of fluid from the positioning means 110over the line 115 so as to force the connecting rods 104 away from thepositioning means 110 a predetermined length, thereby causing the piston88 to be moved the predetermined length towards the plane S. When thepump 112 does not supply fluid to or receive fluid from the positioningmeans 110, the rods 104 and thus the pistons 88, are maintained in placeat the same distance away from the plane S.

The piston positioning means 110 is of a sufficient size and suitablypositioned within the assembly 48 and the connecting rods 104 are ofsufficient length to permit that the piston positioning means 110 cancontrollably retain the connecting rods 104 when the connecting rods 104are positioned such that: (i) the end surface 38 is in the same plane asthe end surface 92 of the piston 88; and (ii) the piston 88 is extendedinto the gap region 42 to a maximum extent, which would constitute amaximum level of magnetic zoom for the magnet assembly. When at least aportion of the end surfaces 92 of the pistons 88 are extended into thegap region 42, the pole separation distance is the distance between theend surfaces 92 of the pistons 88 and the gap distance is the distancebetween the facing surfaces of the supports 100 which are mounted on therespective surfaces 92. The gap distance at the maximum level ofmagnetic zoom is about 12 inches.

It is to be understood that the assembly 48 can contain other suitablemechanical devices for controllably positioning the connecting rods 104at different positions in the gap region 42 in relation to the plane Sin accordance with present invention, such as, for example, a pneumaticpiston positioning system.

The dimensions of the piston 88 and the hollowed region 86 provide thatthe outer surface of the piston 88 is, preferably, in substantialcontact with the surface of the pole 22 which defines the region 86.Also, when at least a portion of the piston 88 is within the aperture44, the outer surface of the piston 88 which is within the aperture 44is preferably in substantial contact with the surface of the polesupport 12 which defines the region 44. The diameter W of the piston 88,the cylindrical hollow region 86 and the aperture 44 is suitably set todefine a smaller size volume region of the patient 54 which is toundergo higher resolution scanning in the magnetic zoom mode and thestrength of the magnetic field at the smaller size region. The diameterW, preferably, is about 24 inches and can be larger or smaller, asdesired.

It is to be understood that the piston 88 may assume other shapes, suchas an elliptical or rectangular body shape, and that the hollowedregions in the pole and the aperture in the pole support would have acorresponding structure which would ensure close contact between thesurfaces of the piston which face the pole and the pole support and thesurfaces of the pole and the pole support which define the hollowedregion and the aperture, respectively.

In a preferred embodiment, the length L of the piston 88 is sufficientto ensure that when at least a portion of the piston 88 is positionedwithin the gap region 42, the outer surface of the piston 88 contacts alarge area of the surface of the pole 22 which defines the region 86.The length of the piston 88, preferably, provides that when the piston88 vertically protrudes into the gap region 42 to the maximum extent,thereby providing the maximum magnetic zoom, a large flux contact areabetween the facing surfaces of the piston 88 and the pole 22 equal toΠW×t is maintained. This large of a flux contact area maximizes theamount of transfer of the flux that the coil 50 generates and isdirected into the portion of the gap region 42 which is defined betweenthe end surfaces 92 of the respective pistons 88. The quality of theferromagnetic material used in the pole 22 and the amount of fieldstrength required for achieving a predetermined level of scanningresolution in the magnetic zoom mode determines the amount of fluxcontact that would be required.

In a preferred embodiment, the length L of the piston 88 is sufficientto ensure that the piston 88 always contacts, at a minimum, the surfacesof the pole 22 when at least a portion of the piston 88 is extended intothe gap region 42.

The operation of the magnet assembly of FIGS. 1, 2A and 2B including theembodiment of the polar region illustrated in FIG. 3 at both the top andbottom polar regions is, for conciseness, described below for the mostpart with respect to the movement of the piston 88 in the pole 22towards and away from the medial plane S. It is to be understood thatthe piston 88 in the pole 24 is identical in structure and operation tothe piston 88 in the pole 22, and that each of the pistons 88 would movesimultaneously and identically towards and away from the medial plane Sduring magnetic zoom mode operation of the magnet assembly.

Referring to FIGS. 1 and 3, the patient 56 is positioned in the gapregion 42 on the support 54 with the center of an anatomical region ofinterest intersected by the polar axis 26. In the normal mode ofoperation of the magnet assembly, which is ordinarily initiallyperformed, the piston 88 is positioned completely within the pole 22 andthe end surface 92 is in the same plane as the surface 38. The coils 98and 102 in the supports 96 and 100 are both energized for scanning. Thegap distance in the normal mode is the distance between the facingsurfaces of the supports 100 and 96, which are in the same plane, andprovides substantially unimpeded access to the patient 56.

Magnetic resonance images in the normal operation mode are obtained byscanning a relatively large volume region of the patient 56. The largevolume region is defined based on the combined surface area of the endsurfaces 38 and 92 which face the plane S. The scanning resolution isdefined in relation to the entire surface area of the end surfaces 38and 92 and the pole separation distance, which is the distance betweenthe end surfaces 38 and 92 of the opposing poles 22 and 24. The magneticfield strength of the magnet assembly generated by the coils 50 and 52also determines the resolution of the scanning and, for simplicity, itis assumed to be a constant in both the normal and the magnetic zoommodes of operation.

The operation of the magnet assembly in the normal mode may be performedas the patient 56 undergoes surgery in a region near or within theanatomical region being scanned. As the need arises, the physician 62can, via the controller 64, command the magnet assembly to operate inthe magnetic zoom mode.

In the magnetic zoom mode, a higher level of scanning resolution withina smaller volume region of the patient 56, which is defined by thesurface area of the surface 92 which faces the patient 56, is obtained.Upon initially receiving a command to operate in the magnetic zoom moderather than in the normal mode, the controller in the system 58 wouldtransmit control signals to the operating assembly 48, particularly tothe pump 112, to cause the piston positioning means 110 to move thepiston 88 a predetermined distance towards the plane S into the gapregion 42. The positioning means 110 forces the connecting rods 104 and,in turn, the piston 88 into the gap region 42 at smooth and non-abruptincrements based on the amount of fluid that the pump 112 supplies toone of the chambers of the positioning means 110 over the line 114 andthe amount of fluid that the pump 112 receives from the other chamber ofthe positioning means 110 over the line 115. Similarly, the positioningmeans 110 provides that the piston 88 can be retracted from the gapregion 42 in smooth and non-abrupt increments based on the fluidreceived therefrom and supplied thereto by the pump 112 over the lines114 and 115, respectively. Also, in the magnetic zoom mode, the system58 energizes only the coils 102 in the piston 88.

The surgeon 62 can command the system 58 to locate the piston 88 tovarious preset positions within the gap region 42 to achieve respectivehigher levels of scanning resolution, as desired. For example, if thesurgeon 62 desires to view images of the same smaller region of thepatient 56 at various preset levels of increased scanning resolution,the surgeon 62 can command the system 58, via the controller 64, tolocate the piston 88 further into the gap region 42. At a higherscanning resolution level, the pole separation distance is the distancebetween the surfaces 92 of the opposing pistons 88 in the poles 22 and24 with the pistons 88 within the gap region 42. The movement of thepistons 88 into the gap region 42 also decreases the gap distance. Atthe maximum magnetic zoom, the pole separation distance is about 12inches.

If the positioning means 110 malfunctions, such that the positioningmeans 110 cannot controllably retain the rods 104, the stop cylinders109 on the rods 104 would prevent the rods, and hence the piston 88,from moving closer than a predetermined distance away from the plane S.The stop cylinders 109 prevent the rods 104 from emerging from thepiston positioning means 110 beyond a predetermined extent at theapertures where the rods 104 are received. The reinforced supports 119in combination with the assembly 48 can support the weight of the piston88 and the piston positioning means 110. Thus, the patient 56 isprotected from injury which would be caused if the piston 88 of the pole22 accidentally fell onto the patient 56.

The movement of the pistons 88 of the poles 22 and 24 into the gapregion 42 causes magnetic flux to be applied through a volume regiondefined between the surfaces 92 of the opposing pistons 88. The smallerpole separation distance in the magnetic zoom made, in comparison to thenormal mode, provides for an increase in the magnetic field strength atthe region of interest positioned in the gap region 42 between thesurfaces 92. Although at least a portion of the piston 88 protrudes fromthe pole 22 into the gap region 42 in the magnetic zoom mode, the lengthof the piston 88 is sufficient to maintain a sufficiently large area ofcontact with the pole 22. This large flux contact ensures the flux fromthe coil 50 is efficiently transferred into the piston 88 and throughthe smaller pole separation distance of the gap region 42 in themagnetic zoom mode. Further, the smaller gap distance in the maximummagnetic zoom level, in comparison to that of the normal mode, does notsubstantially impede access to the patient 56 by others, such as tointerfere with surgery that is being performed on the patient 56.

The combination of a high-level of flux transference, which the largeflux contact area between the piston 88 and the pole 22 provides, andthe movement of the piston 88 further into the gap region 42 to decreasethe pole separation distance and the gap distance of the magnetassembly, advantageously operates to produce higher magnetic fieldsthrough the smaller region of interest in the form of an increased fluxdensity. The increased flux density in the smaller region of the patient56 provides for a higher resolution scanning within that smaller region,because the detected radiation signals at the antennae 59 for thesmaller scanned region would have a higher radio frequency and a highersignal-to-noise ratio.

In one alternative embodiment, a series of different transmitting andreceiving coils in antennae, each of which is tuned for the frequency ofthe corresponding preset piston location, provides the frequencyappropriate to the preset position of the pistons 88. In anotheralternative embodiment, a single receiving and transmitting coil orantennae can be tuned to multiple frequencies.

The radiation signals that are detected when the magnet assembly isoperated in the magnetic zoom mode are processed to obtain magneticresonance images in a manner similar to that performed to obtainmagnetic resonance images when the magnet assembly is not operated inthe magnetic zoom mode.

In one embodiment, when the microcontroller in the system 58 receives acommand for moving the pistons 88, the microcontroller automaticallyde-energizes all of the coils, including the coils 50, 98 and 102, andthen moves the pistons 88 to the next desired position with respect tothe plane S, and then re-energizes all of the coils. Alternatively, thepistons 88 can be moved in a full field condition, while all of thecoils are energized.

In one preferred embodiment of the magnetic zoom mode, the radiofrequency coils 59 can be disposed in greater proximity to the region ofinterest being scanned to obtain further improvements in the scanningresolution.

In another preferred embodiment, a plurality of hollowed regions andapertures can be defined in the poles and the pole supports to receive aplurality of pistons, respectively, in a magnet assembly, in accordancewith the present invention, to provide that a plurality of smallervolume regions of a patient can be scanned individually, or incombination, at higher scanning resolution levels in the magnetic zoommode.

FIG. 4A illustrates an alternative embodiment of the top polar region ofthe magnet assembly shown in FIG. 3 including another ferromagneticpiston 118 which provides for adjusting the magnetic fields generatedwhen the magnet assembly is operated in the magnetic zoom mode. Likereference numerals are used to refer to elements having similar and,preferably, identical structural and functional characteristics as thosedescribed above in connection with FIG. 3.

Referring to FIG. 4A, the body portions 28 and 30 of the pole 22 arehollowed axially symmetrically about the polar axis 26 to a constantdiameter Y to define a hollow: cylindrical volume region 116 within thepole 22. The hollowed region 116 extends from the surface 28 to thesurface of the pole support 12 adjacent to the body portion 28 and has alength equal to t. The aperture 44 defined in the pole support 12 alsohas been hollowed about the polar axis 26 to the same constant diameterY. The inner surface of the pole support 12 which defines the aperture44 and the inner surface of the pole 22 which defines the region 116are, thus, aligned with each other.

The hollowed region 116 contains a piston 118 which is comprised offerromagnetic material. The piston 118 is in the shape of a hollowedcylinder bounded lengthwise by an end surface 120 which faces theassembly 48 and an end surface 122 which faces the gap region 42. Theouter surface of the piston 118, which extends between the end surfaces90 and 92, has a constant diameter equal or substantially equal to Y.The inner surface of the piston 118, which extends between the endsurfaces 120 and 122 and defines a hollowed region 86A within the piston118, has a constant diameter equal or substantially equal to W. Thedistance between the end surfaces 120 and 122 is equal to M. Thus, theouter surface of the piston 118 has a constant diameter which issubstantially equal to the diameter Y of the region 116 and the aperture44.

Ends 128 of two connecting rods 130 are each rigidly secured to the endsurface 120 of the piston 118. The connecting rods 130 extend from theend surface 120, through the aperture 44 and are connected at oppositeends 132 to a second piston positioning means 134 which is contained inthe operating assembly 48. The rods 130 further include stop cylinders135 at the ends 132 which are similar in structure and operation as thestop cylinders 109. Also, the piston positioning means 134 is rigidlyconnected to the assembly 48 by reinforced supports 121 which aresimilar in structure and operation to the supports 119. The pump 112 iscoupled to the piston positioning means 134 over the lines 136 and 137.The piston positioning means 134 is similar in structure and operationto the piston positioning means 110.

The combination of the piston positioning means 134, the actuating means112 and the lines 136 and 137, like the combination of the pistonpositioning means 110, the actuating means 112 and the line 114,constitutes a conventional hydraulic positioning device that iscontrollable by signals that a microcontroller, such as themicrocontroller in the system 58, supplies to the positioning means 134.Based on the control signals supplied to the pump 112, the pump 112supplies a predetermined amount of fluid under pressure to and receivesa predetermined amount of fluid from the piston positioning means 134over the lines 136 and 137 to hold the rods 130 stationary or to movethe rods 130 towards or away from the medial plane S. Thus, the assembly48 provides for independent control of the positioning of the piston 118in relation to the plane S.

The hollowed region 86A of the piston 118 contains the piston 88therein. Therefore, the pole 22 includes a pair of axially symmetricconcentric pistons. The outer surface of the piston 118 is, preferably,substantially in contact with the surface of the pole 22 which definesthe region 116. Also, the outer surface of the piston 88 is, preferably,substantially in contact with the surface of the piston 88 which definesthe region 86. When at least a portion of the piston 118 is within theaperture 44, the outer surface of the piston 118 which is within theaperture 44 is substantially in contact with the adjacent facing surfaceof the pole 22 which defines the region 44. Consequently, the facingsurfaces of the pistons 88 and 118, the pole 22 and the pole support 12provide a low reluctance path for flux.

The diameter W of the piston 88 and the width of the end surfaces 120and 122 of the piston 118, which is defined as the difference between Yand W are suitably set to define the size of the smaller volume regionof the patient 56 which is to undergo higher resolution scanning. Thevalues for Y and W are selected to provide for suitable adjustment ofthe uniformity of the magnetic field that passes through the gap region42 between the facing surfaces of the support 100 mounted on thesurfaces 92 at the higher resolution scanning levels attainable in themagnetic zoom mode. The diameters W and Y, preferably, are about 24 and30 inches, respectively.

It is to be understood that the piston 118 and the hollowed region 86Awhich it defines may assume other shapes, such as an elliptical orrectangular box. The hollowed regions in the poles and the apertures inthe pole supports would have a corresponding structure to receive thepistons 88 and 118 which also would have corresponding structures. Thiscorrespondence in structure would maintain as close contact between thefacing wall surfaces of the pistons and the poles as possible.

The piston positioning means 134 is of a sufficient size and suitablypositioned within the assembly 48 and the connecting rods 130 are ofsufficient length to permit that the piston positioning means 134 cancontrollably retain the connecting rods 134 when the connecting rods 134are positioned such that: (i) the face surface 122 is aligned in thesame plane as the surface 38 of the pole 22; and (ii) the piston 118 isextended into the gap region 42 to a necessary extent in relation to theextent that the piston 88 is extended the gap region 42 to providesuitable adjustment of the magnetic field in the magnetic zoom mode ofoperation for the magnet assembly.

In operation of a magnet assembly of the present invention including topand bottom polar regions as shown in FIG. 4A in the magnetic zoom mode,the piston positioning means 134 independently controls the position ofthe piston 118 in relation to the plane S to adjust the magnetic fieldthat is applied through the smaller volume region of the patient inaccordance with the level of magnetic zoom applied. The piston 118 actsas a tunable shim bar for the piston 88. The amount that the piston 118is moved towards or away from the plane S in relation to movement of thepiston 88 towards or away from the plane S to adjust the magnetic fieldstrength is determined automatically based on values stored in thememory of the system 58, such as in a ROM lookup table. These values arecalculated to account for the different field strengths that the coilsof a magnet assembly generate and the increased field strength that isobtained when the piston 88 is moved a predetermined distance into thegap region 42.

FIG. 4B shows an alternative embodiment of the polar region shown inFIG. 4A which has the same components, except that the hollowed region116 in the pole 22 and the aperture 44, although aligned with eachother, are not axially symmetric about the polar axis 26. Thisarrangement of the pistons 88 and 118 translates the region of interestin which scanning in the magnetic zoom mode is performed toward the edgeof the pole 22 and away from the polar axis 26 or the pole center. Anoff-pole center magnetic zoom feature may be desirable in particularsurgical applications where scanning of a first region in the normaloperation mode of the magnet assembly is desired and scanning of asecond smaller region at a higher scanning resolution and at a region ofthe patient which is shifted from the polar axis 26 is also desiredwithout having to the move the patient on the support 80 or the support80 itself. This feature is particularly suitable for delicate surgicalprocedures which require that the patient is maintained absolutelystable throughout and for which it is desired to scan a smaller regionof the patient in the magnetic zoom mode and also to scan a largerregion, which is not concentric with the smaller region, at a lowerscanning resolution in the normal mode of operation.

In another aspect of the invention, magnetic zoom capability in a magnetassembly is provided by coupling an electromagnetic magnetic zoom meansto each of the poles. It is also to be understood that theelectromagnetic magnetic zoom means can be coupled to each of the polesalone or in combination with a suitable mechanical magnetic zoom meanswhich is also coupled to each of the poles. In one preferred embodiment,the electromagnetic magnetic zoom means may be superconducting.

FIG. 5A illustrates an alternative embodiment of the top polar region ofa magnet assembly as shown in FIG. 2B including an electromagneticmagnetic zoom means 146. Referring to FIG. 5A, the pole 22 has the samestructure as described above in relation to the embodiment of FIG. 3,except that the hollowed region 86 is completely filled withferromagnetic material and the surface 28 also includes the surfaceportion of the filled hollowed region which faces the medial plane S. Aninsulative support 140 is mounted to the surface 38. The support 140contains magnetic field gradient coils 142 which have the same structureand operate in the same manner as the coils 98 in the support 100,described above. The support 140 includes several sets 143 of threadedrecesses 144 in the surface which faces the gap region 42, as moreclearly shown in FIG. 6, which is a plan view of the surface of thesupport 140 which faces the gap region 42. The sets 143 of recesses 144are dispersed on the surface of the support 140 which faces the gapregion 42.

The electromagnetic magnetic zooms 146 is a cylindrical disc support146. The support 146 includes threaded apertures 148 arranged in thesame spatial configuration as the recesses 144 of one of the sets 143 ofthe recesses 144. Threaded ferromagnetic or steel bolts 150, which arethreaded through the apertures 148 and into one of the sets 143 of therecesses 144, securably mount the support 140 to the support 146 andcause the respective facing surfaces to be in close contact with eachother. The plurality of the sets 143 of the recesses 144 allows that thesupport 146 can be mounted at different locations on the support 140 inrelation to the polar axis 26.

The support 146 further comprises high density superconducting coils 152contained in cryostats 154 which are arranged in the support 146 in amanner well known in the art. The coils 152 may be circular, ellipticalor rectangular in shape. The coils 152 determine the thickness of thesupport 146. The support 146 further includes a suitable electricalsignal coupling means (not shown) that allows for connection to thesystem 59.

In operation, when magnetic zoom operation is desired, the physician 62or another attendant initially secures the support 146 to the support140 at a selected position in relation to the polar axis 26 by screwingthe steel bolts 150 through the apertures 148 and into one of the sets143 of the recesses 144. The set 143 that is selected would oppose aregion of the patient 56 for which scanning at a higher resolution isdesired. When the system 56 receives a command to operate in themagnetic zoom mode, the microcontroller provides that a current isinitially supplied to the coils 152 to bias the cryostats 154. Whensuitably powered by the bias current, the coils 152 significantlyincrease the magnetic field strength through the gap region 42 and avolume region of the patient defined by the surfaces of the supports 146which would be coupled to each of the poles 22 and 24 and face the planeS. The gap distance for this embodiment of the magnet assembly is thedistance between the facing surfaces of the supports 146. This gapdistance, like the gap distances for the embodiments of the magnetassemblies operated with magnetic zoom and discussed above, does notsubstantially impede access to the patient by others.

In one preferred embodiment, a plurality of electromagnetic zoomsupports 146 can be mounted on the support 140 simultaneously inaccordance with the present invention, and one or more of the supports146 can be utilized to provide higher scanning resolutions at regions ofthe patient 56 which face the faces of the pairs of the supports 146,respectively.

In an alternative embodiment, shown in FIG. 5B, the support 146 can havean increased thickness to provide that the coils 152 are positionedcloser to the patient 54, a region of which would be positioned in theplane S. The positioning of the coils 152 closer to the patient 54increases the magnetic field strength through the smaller region ofinterest defined by the support 146 and narrows the gap distance withinthe gap region 42. The support 146 also may contain ferromagneticmaterial to further increase the magnetic field strength through thesmaller region being scanned.

In still another alternative embodiment, the support 146 may be formedonly from ferromagnetic material or permanent magnet material and notinclude the cryostats 154 containing the coils 152. The support 146would be attached to the support 140 in the same or similar manner asdescribed above in FIG. 5A or FIG. 5B using the bolts 150. The thicknessof the support 146 would determine the increase in the scanningresolution obtained for a region of the patient defined by the surfacearea of the surface of the support 140 which faces the plane S.

Thus, operation in the magnetic zoom mode operation can be achieved byattaching an identical ferromagnetic structure to the facing surfaces 38and 40 of the poles 22 and 24, respectively, as desired, so that thestructure extends a predetermined distance into the gap region 42.Alternatively, an electromagnetic zoom means can be coupled to thesurface of a ferromagnetic structure which faces the gap region 42,where the ferromagnetic structure is removably attachable to the surfaceof the polar region facing the plane S, to provide for even higherresolution scanning.

FIG. 7 shows a further preferred embodiment of the magnet assembly asshown in FIG. 3 and including an electromagnetic magnetic zoom meanscoupled to the piston 88. Referring to FIG. 7, the support 100 issuitably modified to include recesses 100A in the surface of the support100 which faces the gap region 42. The recesses 100A are disposed in thesupport 100 so that they can be aligned with the apertures 148 in thesupport 146. The magnet assembly further includes an RF coil assemblysupport 160 containing RF receiving and transmitting coils or antennaewhich are linked (not shown) to the system 58. The RF support 160includes aperture 162 which are disposed so that they can be alignedwith the apertures 148 in the support. The configuration of the recesses100A, the apertures 148 and the apertures 162 provides that the support146 can be mounted to the support 100 and the support 160 can be mountedto the support 146 using the bolts 150. In a preferred embodiment, thesupport 146 can be used interchangeably in a magnet assembly whichincludes the piston 88 as shown in FIG. 7, and in a magnet assemblywhich does not include a piston coupled to each of the poles, as shownin FIG. 5A.

FIG. 8 shows still a further embodiment of the magnet assembly, as shownin FIG. 2B, including electromagnetic magnetic zoom means 147A and 147Bfor increasing magnetic field strength in the gap region 42. The poles22 and 24 have the same structure as described above in relation to theembodiment of FIG. 3, except that the hollowed regions 86 are completelyfilled with ferromagnetic material and the surfaces 38 and 40 alsoinclude the surface portion of the filled hollowed region which facesthe medial plane S. The insulative supports 140 are mounted to thesurfaces 38 and 40, respectively, as above.

The electromagnetic magnetic zoom means 147A and 147B comprise identicalcylindrical discs, which are independent and separate structures fromthose structures which comprise the polar regions. Flexible support arms149 attach the discs 147A and 147B to, for example, the bed support 54.The support arms 149, alternatively, can be secured to the floor 66 ofthe room 10. The flexible support arms 149 can be positioned to providethat the discs 147A and 147B can be positioned at a plurality ofpositions in relation to the patient 56 and the polar axis 26. The discs147A and 147B, preferably, can easily be positioned symmetrical aboutthe plane S.

The discs 147A and 147B comprise high density superconducting coils 152contained in cryostats 154 which are arranged in a manner well known inthe art. Suitable electrical signal coupling means (not shown) link thediscs 147A and 147B to the system 58 to provide for energization of thecoils 152 therein. When the coils 152 in the discs 147A and 147B areenergized, a higher level of scanning resolution of a volume region ofthe patient 56, which is defined between the facing surfaces of thediscs 147A and 147B, is obtained.

Consequently, a magnet assembly in accordance with the present inventioncan provide for higher resolution scanning of a smaller region of thepatient in the gap region, in comparison to the region that is scannedand the resolution of scanning that is obtained in the normal modeoperation, by coupling a mechanical or electromagnetic magnetic zoommeans, or both, to each of the poles to face the other pole and at adesired position in relation to the polar axis 26, or by positioningindependent electromagnetic magnetic zoom means in the gap regionproximate a desired region of the patient.

1. A method for conducting a medical procedure, comprising: conducting afirst magnetic resonance imaging scan of at least a first portion of apatient in an imaging volume of a magnetic resonance imaging system at afirst magnetic flux density; conducting the medical procedure;redistributing ferromagnetic material with respect to the imaging volumefrom a first distribution to a second distribution to change themagnetic flux density in a portion of the imaging volume to a secondmagnetic flux density; and conducting a second magnetic resonanceimaging scan of a second portion of the patient at the second magneticflux density.
 2. The method of claim 1, wherein the second magnetic fluxdensity is higher than the first magnetic flux density.
 3. The method ofclaim 2, wherein the first magnetic resonance imaging scan is of a firstportion of the patient which is larger than the second portion of thepatient and the second portion of the patient is contained within thefirst portion of the patient.
 4. The method of claim 1, wherein themedical procedure is a surgical procedure.
 5. A method of conducting amedical procedure, comprising: conducting a medical procedure on asubject within an imaging volume of a magnetic resonance imaging system;positioning a magnetic flux emitting means in the imaging volume toincrease the magnetic flux density in a portion of the imaging volume;and performing a magnetic resonance imaging scan of a portion of thesubject in the portion of the imaging volume at a resolution higher thanthe resolution of a scan obtainable without the magnetic flux emittingmeans in the imaging volume.
 6. The method of claim 5, wherein themagnetic flux emitting means comprises ferromagnetic material, themethod comprising: positioning the ferromagnetic material in the imagingvolume.
 7. The method of claim 6, wherein the imaging volume is definedby opposing ferromagnetic poles and the ferromagnetic material is atleast one block of ferromagnetic material embedded within and movablewith respect to at least one respective ferromagnetic pole, the methodcomprising positioning the magnetic flux emitting means in the imagingvolume by: moving the ferromagnetic material out of the at least oneferromagnetic pole, into the imaging volume.
 8. The method of claim 7,comprising selectively moving the ferromagnetic block to a predeterminedposition with respect to the imaging volume to achieve a predeterminedmagnetic flux density.
 9. The method of claim 7, wherein the second polecomprises a second movable block of ferromagnetic material, the methodfurther comprising positioning the magnetic flux emitting means in theimaging volume by: moving the second block of ferromagnetic materialinto the imaging volume.
 10. The method of claim 7, wherein theferromagnetic block has a face facing the imaging volume and at leastone electromagnetic coil is provided adjacent to the face, the methodfurther comprising positioning the magnetic flux emitting means in theimaging volume by: moving the block and the at least one coil into theimaging volume and energizing the at least one electromagnetic coil tofurther change the magnetic flux density in a portion of the imagingvolume.
 11. The method of claim 7, wherein the first pole comprises asecond movable ferromagnetic block, the method comprising: moving thesecond ferromagnetic block with respect to the imaging volume; andconducting magnetic resonance imaging of at least a third region ofinterest of the patient at a third resolution different than the firstresolution.
 12. The method of claim 7, wherein the first pole furthercomprises a movable piece of ferromagnetic material surrounding theblock, the method further comprising positioning the magnetic fluxemitting means in the imaging volume by: moving the piece offerromagnetic material into the imaging volume.
 13. The method of claim5, wherein the magnetic flux emitting means comprises at least oneelectromagnetic coil supported by a support, the method comprisingpositioning the magnetic flux emitting means in the imaging volume by:coupling the support to the first pole face.
 14. The method of claim 13,wherein the first pole comprises gradient field coils within a secondsupport coupled to the first pole face, between the first support andthe first pole face, the method further comprising: connecting the firstsupport to a surface of the second support.
 15. The method of claim 14,further comprising: connecting a plurality of supports to the secondsupport, each of the plurality of supports supporting at least oneelectromagnetic coil; and selectively energizing selected coils.
 16. Themethod of claim 5, further comprising positioning the magnetic fluxemitting means in the imaging volume by: supporting the magnetic fluxemitting means in the imaging volume, separate from the poles.
 17. Themethod of claim 16, comprising: supporting the magnetic flux emittingmeans by mechanical arms.
 18. The method of claim 17, wherein themagnetic flux emitting means comprises at least one electromagneticcoil.
 19. The method of claim 17, wherein the magnetic flux emittingmeans comprises ferromagnetic or magnetic material.
 20. A method forconducting a medical procedure, comprising: conducting a first magneticresonance imaging scan of at least a first portion of a patient in animaging volume of a magnetic resonance imaging system at a firstmagnetic flux density; positioning a coil in the imaging volume, afterconducting the first magnetic resonance imaging scan; conducting themedical procedure; changing the magnetic flux density in a portion ofthe imaging volume to a second magnetic flux density by a magnetic fieldemitted by the coil; and conducting a second magnetic resonance imagingscan of a second portion of the patient at the second magnetic fluxdensity.